This invention relates generally to machines and procedures for incremental printing of text or graphic images; and more particularly to removal of artifacts in liquid-colorant printing that constructs such images from individual colorant spots, formed in a two-dimensional pixel array on a printing medium such as paper, transparency stock, or other glossy media.
Incremental printing with liquid-base colorants is subject to several very subtle but undesired image defects. Although these arise in generally understood ways from operation of the associated mechanical components, they have nonetheless been very resistant to corrective action.
Many forms of incremental printing operate by creating inkdrop swaths. These swaths are formed in successively stepped positions, by iterated relative motionxe2x80x94along a print-medium advance directionxe2x80x94between the medium and an inking device. Such swath-based printing systems may be of a scanning type, operating by repeated operation of the inking devices across the medium, or a pagewide swath-height array type.
Some artifacts, even though due to different apparatus phenomena, are often designated by the generic term xe2x80x9cbandingxe2x80x9dxe2x80x94meaning that they usually present the appearance of subtle bands, stripes or striations. This is particularly true for swath-based devices, or scanning devicesxe2x80x94although artifacts of interest with respect to preferred embodiments of the invention are not limited to such systems.
The reason for this commonality with respect to banding artifacts is that almost any cyclical or near-cyclical perturbation of inking operations creates some form of correspondingly periodic visible pattern. Merely by way of example, features that appear in generally the same elevation across each swath naturally tend to produce a visual effect at the spatial periodicity of the printing-medium advance.
When printing with a single layer of swaths fitted edge-to-edge, that periodicity is in principle equal to the swath height. (Such an edge-to-edge arrangement is obtained through use of a so-called single-pass printing mode, for a scanning systemxe2x80x94or more generally a single-inking-installment mode, to encompass a pagewide swath system.)
In plural-installment modesxe2x80x94i.e., with overlapping swathsxe2x80x94the spatial periodicity of the banding is finer. In these cases the print medium usually advances by some fraction of the swath height, giving rise to the finer banding.
Some types of bandingxe2x80x94and, more generally, artifacts addressed by some forms of the present inventionxe2x80x94are not fundamentally swath-related at all. Again merely by way of example are those artifacts arising from tiled relatively small printmasks (discussed below).
Such artifacts are most conspicuous in midtone regions of an image, where there is modulation range in either direction to exhibit extremely subtle patterns. Other artifact types, for example those associated with slight overinking effects (also discussed below), are instead most conspicuous in darker regions where colorant liquid effects can dominate.
(a) Boundary bandingxe2x80x94One of the major contributors to banding is a thin, darker line that appears along one or usually all edges of a printed field. This is the so-called xe2x80x9cboundary bandingxe2x80x9d; however, more generally it should be regarded as a boundary artifact.
In a swath-based system, some manifestations of this type of artifact appear to be concentrated particularly where two swath edges abut, or nearly abut. Boundary artifacts are very hard to removexe2x80x94especially when printing in swaths, and especially at a low number of passes or installments.
Normally they appear only in image regions that are rather highly saturated calorimetrically. Such colorimetric saturation giving rise to boundary artifacts, however, occurs in either:
a single primary colorant (ordinarily cyan, magenta, yellow and black), or
a composite color formed from combinations of those colorants in various proportions.
Accordingly this localization of the banding is believed to be a liquid-loading effectxe2x80x94coalescence of the liquid in adjacent inkdrops, concentrated by surface tension at the edges of the just-deposited liquid field.
Boundary artifacts are hard to attack when associated with heavy inking in one primary. This difficulty is exacerbated when the overinking takes the form of an aggregation, with only modest contributions from plural colorants as in the case of composite colors.
Another form of bandingxe2x80x94along horizontal boundaries onlyxe2x80x94arises from swath-height error (xe2x80x9cSHExe2x80x9d, and sometimes xe2x80x9cSWExe2x80x9d) rather than coalescence. It will be discussed below.
(b) Other swath-associated artifactsxe2x80x94One distinctly different kind of image defect, although it too strongly affects contiguous-swath abutments, relates to swath-height error. This type of error usually occurs when nominal relationships between swath height 135 (FIG. 1A) and printing-medium advance 133 fail.
In the nominal relationship, when the effective pen height 135 just matches the advance distance 133, swaths 131, 132 abut neatly 134. For nominal advance, such relationships are maintained when ink-discharging nozzles near the inking-array edge are pointed straight toward the printing medium 130 along a normal to the surface.
When those end nozzles instead point outboard or inboardxe2x80x94along the print-medium advance directionxe2x80x94such misdirections cause the swath to be taller 135xe2x80x2 or shallower 135xe2x80x3, respectively, than its nominal height 135. In the former case, for a nominal print-medium advance stroke 133, excess lengths 136, 137 at top and bottom, respectively, of overlong adjacent swaths 131xe2x80x2, 132xe2x80x2 then overlap slightly. The overlap forms a dark line 134xe2x80x2 (FIG. 1B) along the swath boundary.
In the opposite case of inboard-pointing end nozzles causing shallower swaths 135xe2x80x3, the foreshortened regions 138, 139 at top and bottom, respectively, of the undersize adjacent swaths 131xe2x80x3, 132xe2x80x3 fail to abut at all. The failure 134xe2x80x3 to abut, leaves a white line 134xe2x80x3 (FIG. 1C) between the swaths.
In practice, these conditions arise also with nominal swath height, for short stroke and long stroke respectively. (If the only problem is inaccurate stroke, however, then correction is straightforward and easy.)
Through fine adjustment of the advance stroke, this kind of mismatch between stroke and swath height can be hidden, for some one particular magnitude of nozzle-direction error, but not entirely cured. The previously introduced patent documents of Cluet, Donovan and Doval, Vilanova ""499 and particularly Subirada ""652, introduce various techniques for attacking this problemxe2x80x94and other errors related to nozzle health, taken up shortly.
Generally speaking such techniques also require printing and measurement of test patterns designed to reveal details of the banding characteristics to be overcome. Measurements of this sort are facilitated by apparatus and methodology introduced in several related patent documents, particularly those of Baker, Bockman, Borrell (Ser. No. ""858), Soler, Subirada and Vilanova ""207xe2x80x94as well as the three others noted in the preceding paragraph.
Unfortunately, however, stroke adjustment either foreshortens or lengthens, respectively, the overall image. The image is typically made a significant fraction of one percent too tall or short.
Worse, such a corrective tactic cannot restore image detail that is either blurred or lost, respectively, as the abutment region is distorted in height relative to areas within the swath. Even worse yet is the inability of such strategies to accommodate more than just one particular magnitude of directional error, when in fact each color in an image is printed from a respective different inking arrayxe2x80x94i.e., printhead, or so-called xe2x80x9cpenxe2x80x9d.
For instance suppose that in a particular printer one of these pens prints, in one color, a swath 131xe2x80x3 that is 0.3% too shallow 135xe2x80x3xe2x80x94while another pen makes, in a second color, a swath 131xe2x80x2 that is 0.4% too tall 135xe2x80x2. These two arrays, and their respective two color swaths 131xe2x80x3, 131xe2x80x2, are intrinsically out of register with each other by approximately 0.7%. Any attempt to adjust the stroke 133 to hide swath-height lengthening error 136, 137 or foreshortening error 138, 139 in either of the two colors must necessarily worsen the effect for the other.
In a modern system there are at least four pens. The likelihood of significant mismatch between two is accordingly sizable. Although in principle pens can be sorted into matched sets, doing so increases costxe2x80x94and in any event the ink is generally exhausted from one pen faster than another, so that the practical usefulness of such an approach is limited.
These problems are particularly severe in single-pass (single-installment) printing modes. For instance abutment failure 134xe2x80x3 between two swaths in a particular color leaves an unprinted strip all the way across the image.
If other colors happen to be unused in that region of the image, that unprinted strip is white. The effect is often very conspicuous even if other colors are present, especially since the unprinted strip repeats at intervals equal to the advance stroke. Like boundary artifacts, this type of error is maximally obtrusive when occurring in a highly saturated field of a dark color.
Another type of artifact can arise from an error type that is related to swath-height error: nozzle pointing errors of uncorrelated magnitude and sign, in the advance axis but within the swath rather than at the ends. As adjacent nozzles in different segments along a nozzle column can point either toward or away from each other, results typically include both underinked and overinked strips, respectively.
Thus, like swath-height error but unlike boundary artifacts, these internal pointing errors can create faded or unprinted zones as well as overly dark zones. Unlike both the boundary artifacts and swath-height errors discussed above, these pointing errors naturally are not localized at swath boundaries.
Further, these artifacts may be either isolated from one another or closely grouped, depending entirely on all the conditions of the nozzle array. The entire striation pattern, however, as with those two above-discussed errors, does repeat at intervals equal to the medium-advance stroke.
Still another error source is somewhat related to internally misdirected nozzles: incorrect inkdrop size, or an extreme case of itxe2x80x94total nozzle failure. Inkdrop size can vary due, for example, to low or high firing energy, or to the mechanical characteristics of a heater resistor or a nozzle as manufactured, or to plugging or other degradation of a nozzle through use.
Resulting striations, like those due to internal pointing error, can be either light or dark. They can also be either isolated or clustered within a swath.
Banding artifacts due to swath-height errors, internal pointing errors and inkdrop size errors alike may be classed as xe2x80x9carea fill nonuniformityxe2x80x9d. Such nonuniformity, observed in printmodes with enough passes to conceal boundary artifacts, is mainly generated by a combination of various swath-height errors.
Heretofore a primary strategy for reducing area-fill nonuniformity is adjustment of the advance stroke. The strategy includes selecting an optimum stroke that maximizes image quality. Each pen, however, has its own, different nozzle profilexe2x80x94ideally leading to a specific stroke value for that particular pen.
Under these circumstances, precise compensation of all the pens with just one advance is not possible. The best that can be done is a compromise, and obtaining an ideal compromise requires an advanced and somewhat elaborate procedure.
Such a procedure takes into account characteristics of the printing medium as well as the banding appearance. The procedure also incorporates decisional algorithms to determine the optimized compromise advance for each swath. These requirements also in effect build another kind of compromise between image-quality improvement and throughput loss.
(c) Swath-independent over- and underinkingxe2x80x94These topics relate to excess or inadequate inking that is not localized with respect to a swathxe2x80x94but rather only arises through relatively extreme color-saturation requirements in an image. Accordingly these problems, as will be seen, are only tangentially related to this document.
To achieve vivid colors in printing with liquid inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the liquid basexe2x80x94by evaporation (and, for some printing media, absorption)xe2x80x94and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise: so-called xe2x80x9cbleedxe2x80x9d of one color into another (particularly noticeable at color boundaries that should be sharp), xe2x80x9cblockingxe2x80x9d or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and xe2x80x9ccocklexe2x80x9d or puckering of the printing medium.
In a sense these excess-liquid problems arise because colorant quantities are determined from colors specified in an image file, which are developed without regard for inkingxe2x80x94especially for aggregate liquidxe2x80x94needed to implement those colors. Such color specifications are created by artists, or derived from photographs or other preexisting images, none of which takes into the account the liquid loading associated with all colorants in the aggregate.
Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects. It is helpful to bear in mind, however, that the overall total amount of ink in a region should be actually reduced only as a last resort, since all this ink is what is appropriate for the desired color.
An opposite sort of problem arises when geometrical relationships between ink dots and pixels prevent attainment of linearity in actual color saturationxe2x80x94even though nominally full inking is specified by an image file. This phenomenon, as described at length in the Borrell Ser. No. ""163 document mentioned earlier, results in inadequate apparent visual saturation of colors in image areas that are fully inked.
Once again this particular form of underinking is not at all localized with respect to printheads or swaths. Rather, it transcends such mechanical phenomena, and relates strictly to image color considerations.
(d) Depletion and propletionxe2x80x94The excess-liquid deposition described above is managed by a process called xe2x80x9cdepletionxe2x80x9d, long a familiar one in inkjet printing. This process pauses to correct the absence of aggregate-liquid accounting in the original development of color specifications for an image.
Thus the depletion process typically includes adding the numbers of drops of all colorants at a pixel, and preferably considering the average of such drop count over some practically determined local area. When the resulting quantity exceeds a threshold established through experience with the printing medium, humidity and such considerations related to drying speed, the process may conclude with modification of the derived inking data.
This modification consists of reducing the drop count, usually in such a way as to exert minimal degradation of color accuracy. This condition, however, is a difficult onexe2x80x94since composite-color hue is very sensitive to colorant proportions, and even the brightness of primary colors is sensitive to the amount of colorant deposited.
Concern for such vividness, or at least its linear response in true visual terms, is at the heart of the Borrell ""163 document. Borrell provides for addition of inkxe2x80x94where the averaged liquid loading will permit when needed to realize the visual effects implicit in image data.
Like the over- and underinking conditions that they are designed to correct, both depletion and propletion are essentially swath independent. That is, they transcend swath structures and are localized only with regard to the image itself.
(e) Printmode techniquesxe2x80x94Another useful technique for concealing both banding and excess liquid deposition is laying down in one inking operation by the pen only a fraction of the total ink required in each section of the image. Any artifactsxe2x80x94areas that are either darkened or left unprinted in that inkingxe2x80x94are visually diluted by one or more later inking installments.
Consider, for example, a dark strip. After printing of all the installments, although it remains darker than adjoining color fields, it is much closer to themxe2x80x94especially on a fractional or logarithmic basisxe2x80x94and therefore less conspicuous to the human logarithmic visual response. It typically is printed in only a fraction of the installments (in one installment out of, for example, three or eight), while the adjacent areas are printed with all the installments. Similarly an unprinted strip ordinarily lacks only the ink that should be printed in one installment, being filled in for all the others.
This technique is applicable equally to installments performed by firing a pagewide swath-height array and by scanning a small swath-height printhead across the printing medium. Each installment in a pagewide-array system may be called a xe2x80x9cshotxe2x80x9d, and in a scanning system is usually called a xe2x80x9cpassxe2x80x9d. For simplicity of expression here, the word pass is sometimes used to refer to both.
Thus operation with a single inking installment for each image region may be called a single-pass mode, and with more than one may be termed a plural-pass modexe2x80x94or if more than two a multipass mode. The concept of plural-pass printmodes encompasses multipass operation.
The benefits of plural-pass printmodes are not limited to suppressing the conspicuousness of almost all artifacts, by the visual-dilution effect mentioned above. In addition plural-pass modes tend to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a xe2x80x9cprintmodexe2x80x9d. Heretofore, as reported in several of the patent documents enumerated abovexe2x80x94particularly those of Garcia and Gilxe2x80x94great advances have been made in the design and implementation of printmodes as such, and the data pipelines that implement them.
All those refinements are generally outside the scope of the present document. Some related innovations, however, will be discussed later in this document.
What is particularly important for present purposes is that printmasking cannot cure either problemxe2x80x94either overinking or bandingxe2x80x94in its entirety. Excess-liquid problems reassert themselves as printing throughput increases. Banding, although its conspicuousness is very significantly depressed by multipass printmodes, neverthelessxe2x80x94by the nature of the above-discussed dilution mechanismsxe2x80x94does not disappear entirely.
The remaining band structure is often made extremely subtle, but yet visible and annoyingly persistent. This residual effect often stands out in particular image tonal ranges or colors, and becomes more and more important in a competitive market.
Furthermore multipass printmasking itself obstructs throughput increase, which is now highly prized in that same marketplace, and modern systems are trending back toward low-pass-number printmodes. Hence other solutions must be sought for banding problems.
(f) Other techniques A typical solution for reducing boundary artifacts has historically been to increase the number of passes. This is a trade-off rather than a solution, since printing times are seriously degraded.
Another approach consists of staggering or semistaggering the printheads. Since different printheads do not print their swaths on exactly the same line of pixels, coalescence is not as serious, and the boundary artifacts are much attenuated.
Semistaggering of printheads, though, produces hue-shift banding in bidirectional printing (that is, different portions of the swath show slightly different colors). Fully staggered printheads are not easy to support mechanically, because they require a very wide, flat paper path.
Still another approach, pursued actively in recent years, consists of diminishing the usage of end nozzles of the printhead. Ink is not deposited onto the media as a step function (sudden transition from dry to wet, along the paper-advance axis), but rather is applied as a smoothly growing functionxe2x80x94also called a ramp, nozzle ramp, or nozzle tapering.
This technique is good for a relatively high number of passes (more than about four), but remains insufficient for fewer-pass printmodes (one to four passes). The reason is that the work not done by those end nozzles must be compensated by some backup nozzles, and the number of such backup units that are available is squeezed to the vanishing point as the number of passes is decreased.
The printmasking algorithm becomes more and more constrained with fewer passes and therefore fewer backup nozzles. In addition, steep ramps produce xe2x80x9cbig drop/little dropxe2x80x9d effects (already-inked paper and dry paper display drops differently) that show up as hue-shift banding. So eventually, and for a low number of passes, the state of the art leads to trade-off decisions between boundary and hue-shift banding.
(g) Conclusionxe2x80x94Banding artifacts have continued to impede achievement of uniformly excellent inkjet printing at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is a method for operating an ink-swath system to print an image. The method includes the step of identifying a portion of the image that has high color saturation along at least one swath boundary.
The method also includes the step of printing the image with ink depleted selectively in the identified portion. Here the words xe2x80x9chighxe2x80x9d and xe2x80x9cselectivelyxe2x80x9d are not intended to suggest that ink is diminished only in regions of highest inking. Rather they simply mean thatxe2x80x94within at least some part of the colorant-intensity rangexe2x80x94ink is diminished more where inking is higher.
Similarly, these terms also are not necessarily limited to diminishing of ink to an equal degree throughout a uniform region of high inking. To the contrary, a gradation or spatial hierarchy of ink diminishment is within the scope of the invention as defined by certain of the appended claims. Merely by way of example, in such a gradation more ink may be depleted along the very edge of a swath, and somewhat less may be depleted slightly farther inward from the edge.
Although the term xe2x80x9chighxe2x80x9d may sometimes in a sense be a relative term, in this context it is quite definite: it describes the fundamentally relative nature of preferred embodiments of the invention itself. That is, xe2x80x9chigh inkingxe2x80x9d is associated with xe2x80x9cdiminished selectivelyxe2x80x9d to make clear that, in some part of the calorimetric dynamic range, the method selects higher-inking regions for greater ink diminishment; and lower-inking regions, for lesser diminishment.
The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, by depleting selectively at high-saturation regions that happen to be along a swath boundary, this method incisively attacks the root problem in dealing with boundary artifacts heretoforexe2x80x94since those are precisely the regions subject to such artifacts. All the efforts discussed in a previous section of this document only addressed the boundary-banding problem indirectly.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, if the system has a printmasking function preferably the identifying step includes locating the swath boundary through the printmasking function. (The word xe2x80x9cprintmaskingxe2x80x9d is discussed below in conjunction with the second main facet or aspect of the invention.)
In event that this basic preference is observed, then a subsidiary preference is that the locating include selecting swath positions identified in printmasks. In this case, then in turn it is preferable that the selecting include applying a printmask layer that has bits selectively positioned near the swath boundary.
If the basic preference as to use of the printmasking function is in use, then another subpreference is that the identifying step include determining high color saturation through application of a depletion-localizing mask layer. In this case, then in turn the determining includes operating the depletion-localizing mask layer as a highest-value mask layer. Use of this preference makes it still further preferable that the operating include applying the depletion-localizing mask layer subtractively.
Use of the depletion-localizing mask layer to determine high saturation, as mentioned above, also gives rise to a preference that the determining include applying the depletion-localizing mask layer only for pixels allocated maximum inking in at least one color plane. This preference in turn makes it preferable that the applying include using the depletion-localizing mask layer subtractively.
Another preferencexe2x80x94when the subpreference for use of a depletion-localizing mask layer is in usexe2x80x94is that the locating comprises using that mask layer with bits selectively positioned near the swath boundary. Another basic preference, as to the first independent facet of the invention, is that the identifying step include selection of pixels having maximum inking in at least one color plane.
In preferred embodiments of a second of its aspects, the invention is a method for printing an image with a printer that has a printmasking function. The method includes the step of using the printmasking function to define depletion regions. This second facet of the invention also includes the step of printing the image with ink depleted selectively in the regions.
(The term xe2x80x9cprintmaskingxe2x80x9d usually relates to allocation of inkdrops as among inking passes orxe2x80x94more generally to encompass nonscanning printersxe2x80x94among inking installments. By this definition, a printer that always operates in a single-installment mode needs no printmasking. The great majority of printers that can operate in such a mode, however, are designed to operate in plural-installment modes as well, and therefore do have printmasking subsystems. This facet of the invention therefore can be used even in single-installment operation, if the printer itself has such a subsystem. Moreover even a printer that operates only in single-pass mode can be provided with a printmasking functionxe2x80x94i.e., a subsystem having attributes of a printmasking system other than allocation among installmentsxe2x80x94and thereby enjoy the printmasking-related benefits of the invention.)
In a certain nonstandard sense it may be common to deplete inking along a swath boundaryxe2x80x94but not selectively. That is, whenever a portion of a conventional mask layer (for example, a row along a swath boundary) has more zeroes than ones, in a sense it might be said that depletion is occurring there. On the average, however, the same is found elsewhere in the same mask layer; hence any such depletion is distributed broadly rather than selectively.
Any incidental concentration of mask-based depletion occurs without design, i.e. there is no scheme to concentrate printmask-based depletion at the boundary. Furthermore, statistically a counterbalancing positive increment in inking is found along the swath boundary in other mask layers.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention implements a fundamental recognition that the printmasking function is a window into many mechanical functions of the printer. It enables linking directly to all problems that arise from those functionsxe2x80x94and to curative strategies as well.
Such problems particularly include those related to boundary artifacts, but also other forms of artifact that have their origin in electromechanical components of the printer. In an inkjet printer, merely by way of example, such components may include inkjet nozzles, heaters, firing-signal electronics, scan-encoder subsystems, scanning mechanisms and control electronics, and printing-medium advance mechanisms and their control electronics.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably this facet of the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the using step includes establishing an auxiliary printmask to define the regions.
If this first basic preference is observed, then several other nested subpreferences arise. First, if the system forms the image in swaths of ink it is preferable that the using step include defining depletion regions near at least one swath boundary that is identified through the printmasking function.
Second, if such depletion regions are so defined, then it is also preferable that the defining include establishing the regions near the swath boundary by bits selectively positioned in at least one printmask. This in turn makes it preferable that the using step include defining depletion regions at pixels of high color saturation through application of a depletion-localizing mask layer.
If depletions regions at high-saturation pixels are defined by application of such a later, then it is still further preferable that this application include operating the depletion-localizing mask layer as a highest-value mask layer. This operating, moreover, preferably applying the depletion-localizing mask layer subtractively.
The subtractive application of a depletion-localizing mask layer is also strongly preferred more generally. For example such subtractive application is preferred relative to the basic printmasking function, and again in event of the auxiliary-mask establishment, and so forth.
Another basic preference is that the method further include the step of measuring test-plot optical-density nonuniformity arising from any or all of these types of defects or artifacts or others that produce nonuniformity:
boundary artifacts,
swath-height error,
printing-medium advance error,
bidirectional banding,
minibanding, and
area-fill nonuniformity;
and that the using step include defining, through the printmasking function, depletion regions and magnitudes that compensate for the measured nonuniformity.
In this case a subpreference is that the using step tend to correct all the defects or artifacts for each color plane independently. This particular capability is an extremely powerful result, for prior methodologies and systems were stringently limited to solving these problems by compromise print-medium-advance settings and the like.
Those compromises were required because the artifacts arose independently in the different colorant printheads or so-called xe2x80x9cpensxe2x80x9d. Therefore the compromise settings could do no more than strike a best balance of artifact mitigation among the several color planes, and this best balance was in many or most instances still far from a complete cure for at least one or two of the heads.
Another subpreference arises from the fact that in this second facet of the invention, all the correction is initially taken in the form of depletionxe2x80x94which systematically depresses the overall level of inking. Therefore it is preferable that the method further include the step of raising an ink-limit value to compensate for overall underinking effects of the defining.
In preferred embodiments of a third of its basic aspects or facets, the invention is a method for printing an image with a printer that has a printmasking function. This method includes the step of measuring test-plot optical-density nonuniformity.
It also includes the step of using the printmasking function to define ink-adjustment regions and magnitudes to compensate for the measured nonuniformity. Yet another step is printing the image with ink adjusted selectively in the regions and according to the magnitudes.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention bridges the gap between observed artifacts or anomalies and the precise corrective localizations enabled by other facets of this invention. In essence this facet of the invention takes the form of negative feedback between quantitative measurement of nonuniformity and its compensationxe2x80x94but on a photographically localized basis.
Furthermore, as couched here this aspect of the invention is not limited to a two-stage cure, i.e. subtractive corrections followed by raising an ink limit. It may instead make some subtractive and some additive corrections, as prescribed by the measurements, all in one direct step.
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the measuring step responds to nonuniformity arising from any or all of the types of defects or artifacts enumerated above and, again, others that introduce nonuniformity.
Another preference, as in the case of the second facet, is that the using step tend to correct all the defects or artifacts for each color plane independently. This feature alone represents a major step forward.
In preferred embodiments of a fourth of its aspects, the invention is apparatus for printing an image in inkdrops, subject to mechanical artifacts. The apparatus includes some means for establishing units of ink depletion, prelocalized with respect to such artifacts and manipulated as negative inkdrops.
For purposes of generality and breadth in discussing the invention these means will be called simply the xe2x80x9cestablishing meansxe2x80x9d. In this document the phrase xe2x80x9cmechanical artifactsxe2x80x9d means artifacts in an image that are caused primarily, if not only, by imperfections in the mechanical features of the mechanism as suchxe2x80x94rather than primarily or only by excessive inking requirements arising in the image.
Further, the term xe2x80x9cprelocalizedxe2x80x9d means that potential locations of these depletion units, and the magnitudes of their effects, can be measured before knowing much about the image. (The locations of the depletion units are very loosely described here as xe2x80x9cpotentialxe2x80x9d because the depletion units may not actually be applied, in event the image color saturations are low in those locations.)
It will be understood that there is no such thing, speaking physically or literally, as a xe2x80x9cnegative inkdropxe2x80x9d; the foregoing discussion refers instead to a methodology for manipulating units of ink depletion almost as if they were negative drops. As will be seen in following sections of this document, this methodology simply entails a kind of drop and depletion-unit bookkeeping, or accounting, in which depletion units are offset against inkdrops.
The apparatus also includes a print engine. The engine prints the image with the drops and depletion units.
The foregoing may represent a description or definition of the fourth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, the xe2x80x9cnegative inkdropxe2x80x9d paradigm focuses the correction of boundary artifacts precisely at the pointsxe2x80x94and in the amountsxe2x80x94required. It is an ideal implementation of the potential for cure at the locations so specifically identified through the masking function.
Although the fourth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, when the apparatus is used for printing the image in plural successive inking installments it preferably further includes:
some means for allocating each inkdrop to one of the installments, respectively; and
some means for allocating at least some of the units of ink depletion to one of the installments, respectively.
The engine then prints the image with the drops and units as allocated to the installments.
(Here the term xe2x80x9crespectivelyxe2x80x9d is not intended to suggest that there must be a separate inking installment for each drop; the point is merely that not all the drops or depletion units are allocated to the same one of the installments. An analogous understanding of the word xe2x80x9crespectivelyxe2x80x9d is also intended as to the fifth major facet of the invention discussed below, particularly for the second xe2x80x9calternative preferencexe2x80x9d introduced in that discussion.)
When this preference is observed, then a nested group of preferences comes into play. First, preferably the depletion-allocating means include bits selectively positioned in at least one printmask to define a depletion region with respect to a swath of inkdrops.
If this is the case, then preferably the printmask includes at least one depletion-localizing mask layer, containing the bits, that is invoked only at pixels of high color saturation. In this event, further the depletion-localizing mask layer is a highest-value mask layer.
Another basic preference, as to this apparatus aspect of the invention, is that the depletion units include at least one depletion-localizing mask layer that is invoked only at pixels of high color saturation.
Two alternative basic preferences are that the print engine includexe2x80x94for each colorant that is in usexe2x80x94either a scanning array of inking units, or a pagewide array of inking units. The pagewide array has multiple inking rows for inking a pagewide swath.
Either type of engine also includes a firing system for providing signals to operate the array. Also preferably included are some means for shifting such printing medium along a medium-advance axis between successive inking installments, or successive groups of inking installments, of the array.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: